There has been a trend in recent years for motor vehicle displays to increase in size and to become more complex functionally. There is, however, a limit to the size and complexity of practical automotive displays incorporating numerous separate display devices, in the same area or in overlapping areas within a single display unit, for example, mechanical dials, gauges and warning lights. Dashboard space is also increasingly at a premium in automobiles.
Rather than increasing the number of components, size, complexity and cost of display units, there is a trend for increasing use of liquid crystal display (LCD) devices in automotive dashboard display units, either by increasing the size of a single LCD device or by using multiple LCD devices. Such LCD devices may, of course, also be used in conjunction with other display devices within the same display unit.
Often, there is a desire to maximize the so-called “dark panel” effect, which is the partial or full concealment of display elements, including back-lit LCD devices, when not in use, so that such display elements or their outlines blend into a surrounding background area within the display unit. To the eye, the concealed display devices and their associated display areas then appear dark, even in bright ambient lighting conditions.
An automotive dashboard display unit will normally have a clear or partially absorbing cover sheet spaced in front of the display devices and display areas of the display unit. If partially absorbing, the cover sheet will be visually clear (i.e. non-scattering) and may either have a neutral color such as grey, or may alternatively be tinted with a color for visual appeal. All such partially absorbing cover sheets, whether of neutral color or tinted, will be referred to herein as “neutral density filters”.
Often the cover sheet will be shielded by a protruding bezel or display surround and be angled so as not to direct stray reflections or bright lights or daylight back towards the eyes of the viewer. The effect of ambient light reflected from the cover sheet is then minimised. However, a significant amount of ambient light will still, of course, be incident on the display areas. Although reflective LCD devices may benefit from such reflected ambient light, this disclosure relates to back-lit LCD devices where ambient light is not used to display information displayed by the LCD device. Such ambient light, when scattered or reflected back towards the viewer from the display unit can reduce the contrast of the displayed information by the back-lit LCD device.
Returned ambient light can also illuminate or emphasize the outer surface of the LCD device itself or the edge or border between the LCD device one or more areas bounding or surrounding the LCD display area, whether or not the LCD is active. Such returned light impairs the dark panel effect.
In such circumstances, one way of improving the dark panel effect is to use a neutral density filter above the display area which reduces the intensity of ambient light incident on the display area and also reduces the intensity of any returned light that is scattered or reflected back towards the viewer. Such a neutral density filter is often incorporated within the outermost cover sheet spaced in front of the display devices and display areas of the display unit. Although the neutral density filter can substantially eliminate returned ambient light such that the dark panel effect is maximized, this is at the cost of reducing the intensity of the transmitted light visible to the user from all light sources within the display unit. It has been found in practice that to achieve a good dark panel effect using such a neutral density filter, it is necessary for the filter to have a transmission of about 37% and to achieve an excellent dark panel effect using such a neutral density filter, it is necessary for the filter to have a transmission of about 26%. To compensate, it becomes necessary to increase the luminance of light sources such as those used to illuminate back-lit LCD devices, with a consequent increase in power consumption and cost for the display unit.
This disclosure provides a display unit having an improved dark panel effect while maintaining an adequate contrast ratio and display brightness in a back-lit LCD device within a display unit.